Semiconductor integrated circuits, or chips, are small (usually square or rectangular) pieces of semiconductor material (e.g., silicon or gallium arsenide) with dimensions typically on the order of 2-7 millimeters on a side. Semiconductor chips may contain complex circuitry formed of hundreds of thousands of individual electronic components. Naturally, to utilize such a chip, connections must be made between the chip itself and external circuitry. To facilitate use of the chip and the making of such connections, the chip is usually packaged in a housing equipped with a lead structure incorporating electrical leads, each of which is at one end electrically bonded to the chip and at its other end serves as a connection point to other circuits. Inside the package, these leads are connected to individual terminal sites, known as bonding pads, on the chip. A variety of conventional techniques are available to accomplish this bonding.
A leadframe typically carries only one chip. Thus, interconnections from chip to chip usually are made exteriorly to the chip package. While this provides flexibility in arranging interchip connections, there are situations in which such flexibility is unnecessary and adds to the cost of a product using the chips. Sometimes two chips are designed to be connected together in a particular fashion and advantages would accrue from connecting the chips within a single package, so that two chips could be sold as a single unit. Of course, there are instances when the circuitry on two smaller chips can be combined to be fabricated as a single chip in a single package. This is not possible, however, when the two constituent chips are manufactured using different process technologies.
To assemble two chips on a single leadframe, the chips must be placed side by side on the leadframe's die attach paddle. However, this approach cannot be used with chips which are backside biased, unless their biasing needs are the same under all conditions.
To solve this problem, some companies have removed the usual metal die attach paddle in the leadframe assembly and have replaced it with a dielectric substrate, onto which multiple chips may be mounted. The dielectric substrate is typically a ceramic or resin. The use of a dielectric is necessitated by the requirement to electrically isolate the substrates of the chips, so they may be separately backside biased. This solves the technical problem, but at considerable expense.
Accordingly, it is an object of the present invention to provide an inexpensive technique for assembling multiple integrated circuit chips on a single leadframe.
Another object of the invention is to provide a multiple chip mounting arrangement on a single leadframe which does not require removal of the metal die attach paddle of the leadframe or use of a dielectric substrate as a foundation for mounting the chips.